


You Will Know, With Her Feet Down to the Ground

by Longanimals



Category: Undertale (Video Game)
Genre: 69 (Sex Position), Cheating, F/M, Mating Press, Monster Girl, Muscles, Power Bottom, Robot Sex, Robots, Temperature Play, Vaginal Sex
Language: English
Status: Completed
Published: 2020-04-13
Updated: 2020-04-13
Packaged: 2021-03-02 04:35:27
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,074
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23639179
Author URL: https://archiveofourown.org/users/Longanimals/pseuds/Longanimals
Summary: Undyne and Alphys are in a loving relationship. Unfortunately, they're both bottoms, and Undyne feels a little frustrated with her girlfriend's performance in bed. So, she finds a modification for Mettaton that makes him a living sex toy and gets some well-earned relief.
Relationships: Mettaton & Undyne (Undertale)
Kudos: 10





	You Will Know, With Her Feet Down to the Ground

Undyne tossed and turned, trying to find the right position to fall asleep. She had been doing this for hours with nothing to show for it. She sat up and sighed. Just a few inches to the left, Undyne’s girlfriend, Alphys, was sleeping soundly on top of the blankets. She was so cute when she was sleeping...Her tail was rolled up onto her chest and she was grasping it with her arms. Undyne smiled. She was lucky to have her.

The only problem with their relationship was the sex. Alphys was a very submissive bottom, but Undyne was a power bottom. She never admitted this to her, and felt that if she mentioned it it would only make her insecure. Undyne masturbated a lot, but it wasn’t enough. She needed to be properly fucked by a top.

Silently, she stood up and walked out of the bedroom. She was wearing only a black tank top and a pair of white panties, contrasting with Alphys’s colorful nightgown. The royal guard captain sat down at the office desk, preparing for another sleepless night. She turned on her girlfriend’s computer, hoping to kill the time with some video games. However, a text document was already open. Undyne didn’t want to invade her lover’s privacy, but curiosity got the better of her. It was crammed with diagrams, numbers, and complex science terms. She scrolled through, knowing she was completely out of her league, when something piqued her interest. It was a blueprint for some kind of robotic parts. Most of the pictures looked familiar. They looked like Mettaton and his various forms.

Undyne tried to decipher the scientist’s notes to see if there was anything important, but she simply couldn’t. Just when she was about to close out of the document, she noticed a part that seemed out of place. It was a medium-sized rod. On one end of the rod was a black cap with a heart at the end. Upon closer inspection, she saw that the heart had a small slit in the middle of it. What was this? She took a crack at reading the notes to try and get some context. They weren’t as complicated as the other ones, being a bit more blunt and to-the-point.

“ROBOT PENIS. Length: 8 ¾ inches. Width: 1 ½ inches. Optional attachment to Mettaton. Tube runs through the middle and is connected to a storage bank that holds 50 mL of synthetic semen. Heart shape on top for aesthetic purposes. Artificial nerves are connected to his soul, providing an accurate replication of pleasure. In between the semen tube and the outside are heat generators powered by Mettaton’s internal batteries. Programmable thermostats are connected to the generators, with a minimum heat of 90 degrees Fahrenheit and a maximum heat of 110 degrees Fahrenheit.”

Undyne couldn’t believe this. Had Alphys really built Mettaton to be a sex toy as well as a performer? Maybe this was from before they had started dating, and that’s how she got her rocks off before Undyne came along. She smiled, proud of her girlfriend for doing something so taboo without her knowing or daring her to do it. She kept reading, hoping to find some more juicy information about Alphys’s perverted masturbation habits. She didn’t have to look far, as she soon found another entry with just data.

“SYNTHETIC CUM. Ingredients: Water, Polyethylene Oxide, Titanium Dioxide, Potassium Sorbate, Chocolate Flavoring. Last ingredient is a variable, and I’m still experimenting with other flavors. Substance is safe for consumption, and will not have any adverse effects if released inside a vagina, anus, cloaca, oviduct, etc. Internal cum storage is kept at 98 degrees Fahrenheit at all times to simulate body temperature. Note to self: Find out what actual semen tastes like then try and synthesize it.”

Undyne laughed. She knew better than anyone that the scientist could be deceptively kinky, but this was a whole different level. Mettaton was the symbol of the underground, loved by humans and monsters alike, and the fact that his creator had modded him to be her own personal dildo was a bizarre form of exhibitionism. She thought about Alphys riding her creation until his dick shot out fake cum inside of her…

Before she knew it, Undyne had a hand in her panties, rubbing at her pussy. She was horny before, but reading about Mettaton’s new mods only fanned the flames of her desire. Fuck...she needed an orgasm, badly. A carnal part of her was curious to see what Mettaton’s dick was like, and if the scientist’s imitation could come close to the real thing.

Alphys wouldn’t mind if I took a peek, right? She tried to reason with herself. Slowly, the fish woman moved towards Mettaton’s charging chamber. She opened up the door and saw the robot standing still, his eyes closed and a mess of wires connected to his back. He was already in his EX form from a show the night before. Undyne just stood there, admiring the work of engineering standing right in front of her. He truly was beautiful, even if he wasn’t organic. She decided that she had wasted enough time. She saw a large button that said “ON” and pressed it. Automatic pneumatic systems activated, and all of the wires connecting Mettaton to the wall fell to the floor. His eyes opened, and he struck a pose.

“Hello, darling! Oh, it’s Undyne! How good to see you again,” he blew her a fake kiss, which made her blush a bright blue. “To what do I owe the pleasure at this time of night?”

Undyne swallowed. “Well, y’see, I was digging through Alphys’s notes, and saw that you had some...interesting modifications, and I was wondering if I could see them in action?”

He chuckled mechanically. “You’ll have to specify WHICH modifications, dear. I have many for all sorts of purposes.”

She steeled herself. She was the captain of the royal guard, she wasn’t going to let this faze her!

“I saw you had a robot dick and I want to fuck it!” She blurted.

Her request had taken him completely by surprise. “Oh my...I haven’t thought about that thing in a long time. Alphys used it to sexually relieve herself before you two started dating, and it’s just been collecting dust ever since. I will happily indulge your fantasies. Just give me one moment to prepare.”

Undyne was beside herself in excitement. She was really about to have sex with a robot, no, THE most important robot in the world! Mettaton sauntered over to a drawer, exaggerating his luscious legs with each step. Being a showman was in his programming, and it turns out that being a showman isn’t that different from being a tease. He bent over seductively as he searched for his “attachment”. Undyne knew that he was hamming it up like he always did, but that didn’t make her any less aroused. Her underwear had a distinct dark spot in them from where she was leaking, and she bit her lip in anticipation.

She heard a mechanical *whirr*, then a satisfying *click!*. Mettaton did a sharp, 180 degree turn on one heel and stood proudly, hands on his hips. He looked mostly the same, except now he had the metal rod she saw in the blueprints fastened to his crotch, right below his soul.

She got down on her knees, eyes wide, right in front of it. It looked even more remarkable in person than it did on the computer screen. She grabbed in with a fish-scaled hand. It was as hard as steel…

But not very hot. She remembered reading about a programmable temperature in Alphys’s notes, and wanted to see if she could make it warmer.

“Uh, Mettaton? Can you increase the temperature?”

“Absolutely, dearie. What would you like it to be?”

Undyne thought about it. The maximum temperature was 110 degrees, which was much more than the average. She was cold blooded, and this could potentially be dangerous for her. Then again, Alphys was cold blooded too, and she obviously managed to do it...Fuck it, there was no way she could pass up on a chance like this!

“All the way up! I wanna feel the heat!”

“Oh, my. Feeling bold today, are we?” The robotic incubus giggled and turned a dial on his backside. Her hand was still on his cock, and she could feel the generators getting to work. The metal encasing his dick was warming up, and within seconds, it had reached its peak. It was scorching hot, and her skin sizzled on top of it.

She couldn’t take it anymore. She didn’t even notice that her other hand was idly rubbing at the lips of her cooch, she was so horny. She stood up and started to undress, preparing for the main event. She quickly took off her tank top and soaked panties and discarded them on the floor.

“You look positively marvelous, darling,” Mettaton flirted.

“I know I do!” Undyne said with confidence. She flexed her muscles and grinned. Her metallic lover covered his mouth with a hand and let out a low “Ohhh…” under his breath. At first glance, you wouldn’t suspect her to be a bottom. She had an eyepatch, a mouth full of sharp teeth, and a body rippling with muscles. She had dense B-cup breasts, a taut stomach and a tight ass. There wasn’t a single ounce of fat or loose scaly skin on her body, and goddamn did she know it. Her fingers and toes were webbed like a fish and her skin was a bright shade of blue. Last but not least about her appearance was her pinkish-red hair. Usually it was worn in a ponytail, but right now it hung down to her waist in a natural mess of bedhead.

She laid faceup on the ground, legs spread wide open. “C’mon, you android fucker! Show me what you got!” she taunted.

“Oh, the nerve! I was going to take it easy on you, but it seems like you want to be treated rough?” Mettaton teased, faking the guise of an offended dom.

Undyne liked where this was going, and decided to play along. “You bet I do! Now get down here!”

Mettaton stood still for a second, his eyes blank, as if he was processing something. “Dominant mode: Activated. Registered Safeword: Papyrus,” he said in an uncharacteristically robotic voice. Undyne was a little freaked out and his outburst took her out of the mood a little, but fortunately he got right back in character. Well, at least Alphys had the foresight to include a safeword function. An out of control robotic dom was a scary thought!

The android laid himself down on top of her, staring face to face. He lined up his artificial cock with her pussy, and she mewled at the slight shock of heat from his tip. He put his hands on the floor under her armpits to support himself, and Undyne instinctively wrapped her elbows around his wrists. While his dick was piping hot, the rest of his leaden skin was cool to the touch. He got as close to her fishy ear as he could.

“I’m putting it in now,” he whispered commandingly. It wasn’t a question or a request, it was a statement. This was something that was going to happen. Mettaton inched forward with mechanical precision, making sure to only put in just enough to leave her gasping for more. Centimeter by centimeter, it pushed inside, radiating love throughout her body. She winced, feeling the searing heat touch her walls. He continued his advance at a glacial pace. She wanted nothing more than for him to drive his powerful metal cock inside and destroy her, and he knew this. God, this was just making her more frustrated! She wanted to scream!

After a painstakingly long time, his whole length was inside. The head was poking at her cervix, and it spread warmth to everything it touched. The android moved his head from next to her ear to staring her dead in the face.

“Get ready,” he said with certainty.

Undyne didn’t think that she could have gotten ready, even with all the time in the world. He started jackhammering at her pussy, forcing his entire length in and out at speeds no human or monster could even hope to achieve. Undyne’s eye dilated. This...this is what she had come for.

Her robotic lover stared down at her with a sly smile. She wrapped her scaly hands around his neck and locked her legs around his waist. They were now in a proper mating press. She pulled his face in for a kiss. His cold lips and tongue were the polar opposite of the scorching heat down in her cunt, and she couldn’t get enough of it. His eyes were half-lidded with sadistic joy. She had no idea how programs and circuitry could make him experience what seemed to be real pleasure, but it was so genuine that she could almost believe he wasn’t a robot.

His hydraulic-powered hips continued to hammer away at her cunt. It felt so...unique...She was going to cum soon, and Mettaton could sense it. The instant she climaxed, he hilted inside, and they enjoyed a tender, romantic moment. Undyne moaned into the androids mouth as the electricity of her orgasm coursed through her veins.

He pulled away from her lips. “I’m going to cum,” he announced proudly. She pulled him in as close as she could until it felt like their bodies were one. Mettaton pushed in as deep as he could, and his heart-tipped dick pushed into her womb. He moaned in a masculine way, and she felt it. His cum flooded into the deepest part of her, and her very core was filled with his love. There was so much...her womb was completely filled with his synthetic seed. Eventually, it stopped, and the celebrity pulled out. Undyne put a webbed hand on her belly. It was radiating heat throughout her entire body. She felt so warm…

She needed more. “You up for another round, stud?” She asked flirtatiously.

“I got about 30 more milliliters of cum just begging to be used,” he answered. “So, how do you wanna do it this time?” he asked, leaving it completely up to her.

“Um...How about this? Lay down,” she commanded. He complied, lying faceup. Undyne tried to maneuver on top of him, with her mouth over his dick and her pussy over his face. It was a bit difficult moving with the added weight inside her, but she made it, and they were in position.

“Before we start, would you mind turning your heat down? About 97 degrees?” She had adored the feeling of his dick boiling inside her cunt, but she really didn’t want to burn her mouth.

“Not at all, sweetheart.” He leaned forward a bit and turned the dial on his back down. Undyne listened and heard the generators start to slow down. She gave it an experimental touch. It was a much more comfortable temperature now. She also noticed that Mettaton had given up his soft dom attitude. Maybe he was programmed to reset whenever he came?

In any case, it was time to get started. “You better not slack back there! I expect to feel your tongue all the way inside!” she demanded.

“Oh honey. I NEVER disappoint.”

The effeminate android put his hands on her muscular ass and spread her cheeks. He ran his metallic tongue up and down her slit, and just when Undyne thought he was about to put it in, he slid it back up and started teasing her asshole. She shuddered at his boldness, but didn’t want to interrupt him. He opened up her pucker and pushed his tongue in as deep as he could. He traced circles around the walls. His tongue wasn’t as long as his cock, but goddamn, it made up for it in girth and flexibility. 

Figuring it was her turn, she craned her neck down and started to suck his dick. It was a much more natural temperature now, but that doesn’t mean that it was cold. His shaft still tingled warmly in her mouth. She was a little rusty on her blowjobs, but her tongue game was still some of the best in the underground. She felt where the metal plates of his cock were welded together. Sucking something made of metal was certainly different than something made out of skin, scales or anything in between. She stopped when he reached her uvula, not wanting to activate her gag reflex, and bobbed her head back and forth on the 5 or so inches she could fit in just her mouth.

Mettaton’s tongue felt so fucking good inside her asshole...She started to moan louder around his dick as he picked up the pace. She felt another orgasm coming on, and she forced her ass as far down on his face as she could. She climaxed again, yet this time it was a bit different. This was purely from teasing her ass and mouth, there had been no stimulation to her pussy. It wasn’t a bad difference, but it was still a difference.

Wanting to return the favor, she teased his tip as fast as she could until the android announced “I’m cumming!” She pulled back until just the tip was on her lips, and she felt the flood of delicious cum fill her mouth. Alphys’s notes were right, it tasted like her mouth was being filled with a deluge of chocolate. She swallowed it down as fast as it came, and once he was empty, she was completely full. She pulled her head all the way off his dick and rolled over onto her back. Her insides were now coated with Mettaton’s delicious fake cum. She turned her head to the side and gave him a peck on the cheek. “Thank you. This was so nice.”

“Yes it was, darling. If you or Alphys ever want to shake up your sex lives or just need some relief, I’m always here.”

Mettaton stood up, unscrewed his dick, and carried Undyne back to her bed. He silently tucked her in next to Alphys where they slept peacefully, and he went back to his charging station, preparing for his long day of celebrity life tomorrow.

**Author's Note:**

> If you enjoyed the story, consider following me on [Twitter](https://twitter.com/longanimals_w) (@longanimals_w) and checking out my [carrd](https://longanimals.carrd.co/) for links to my HentaiFoundry, Pixiv and more.


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